1. Field of the Invention
The present invention relates to direct-positive silver halide photographic emulsions comprising high chloride content silver halide grains containing Group VIII metal dopant and which are both reduction and gold surface fogged.
2. Background of the Art
It is known that direct-positive images can be obtained with certain types of photographic silver halide emulsions without previously forming a negative silver image. As described in British patent specification No. 723,019, one photographic emulsion of this type is a photographic emulsion comprising one or more electron-trapping compounds and silver halide grains which are fogged with a combination of a reducing agent and a gold compound or a compound of a metal more electropositive than silver e.g. palladium or platinum.
According to U.S. Pat. Nos. 3,501,305 and 3,501,306, improved photographic direct-positive emulsions of this type are obtained with mono-dispersed direct-positive emulsions, i.e. emulsions the grains of which have substantially the same diameter, more particularly at least 95% by weight or number of the silver halide grains are of a size which is within about 40% of the mean grain size, and with regular grain direct-positive emulsions i.e. emulsions of which at least 80% by weight of the grains have a regular crystal shape. These emulsions are preferably emulsions obtained by combining a low level of gold fogging with a low level of reduction fogging.
Although according to the above U.S. Patents the mean grain diameter of the direct-positive silver halide emulsions may be comprised between about 10 nm and about 2000 nm so that Lippmann emulsions, which have an average grain diameter of less than 100 nm and preferably less than 80 nm, are embraced, the teachings of these patents has not been found to be sufficient to provide direct-positive silver halide Lippmann emulsions yielding upon exposure and development direct-positive images of sufficient overall contrast, sufficient contrast in the highlight areas and sufficient maximum density.
Lippmann emulsions are of particular importance for the preparation of photographic plates or films with high resolution, for use in microphotography and astrophotography, for recording nucleo-physical phenomena, for the preparation of masks in the production of microelectric integrated circuits, for use in holography for high-density data storage, etc.
U.S. Pat. No. 4,082,554 teaches that improved direct-positive images as regards, overall contrast, contrast in the high-light areas and maximum density are obtained upon exposure and development of a direct-positive silver halide Lippmann emulsion comprising reduction and gold fogged silver halide grains of an average grain diameter of less than 100 nm and at least one electron accepting compound when the silver halide grains are fogged with from about 0.07 to about 0.5 milliequivalent per mole of silver halide of a reduction fogging agent and with from about 0.01 to about a 0.1 millimole per mole of silver halide of a gold fogging agent and the silver halide emulsion layer comprises per mole of silver halide more than 2 g and at most about 10 g of electron-accepting compounds.
U.S. Pat. No. 3,945,832 describes a fogged direct positive silver halide emulsion spectrally sensitized with dyes of specified formulae. An emulsion is shown in the Examples which has 80% Cl in the silver halide. No dopants are specifically described in the claims, but are mentioned in the text. No size range is indicated in the Example for the silver halide grains.